If you're looking for stamps, you can get them at any resort and, at the MK, at the City Hall. The resorts will also mail letters and postcards for you; most have a letter drop in the lobby.
If you're looking to send a package, or a box, you can do that at any of the hotel convention/business centers (CSR, BWI, YC/BC, CR, GF). They usually open at 7.
If you need more specific services, you need to go off property.